A cardiac pacing system includes a battery-powered pulse generator and one or more leads for delivering pulses to the heart. The pulse generator includes a source of power. Current pulse generators include electronic circuitry for determining the nature of an irregular rhythm, commonly referred to as an arrhythmia, and for timing the delivery of a pulse for a particular purpose. The pulse generator is typically implanted into a subcutaneous pocket made in the wall of the chest. Insulated wires called leads attached to the pulse generator are routed subcutaneously from the pocket to the shoulder or neck where the leads enter a major vein, typically the subclavian vein. The leads are then routed into a chamber or chambers of the heart. The leads are electrically connected to the pulse generators on one end and are electrically connected to the heart on the other end. Electrodes on the leads provide the electrical connection of the lead to the heart. The leads deliver the electrical discharges from the pulse generator to the heart.
The electrodes typically are arranged on a lead body in two ways or categories. A pair of electrodes which form a single electrical circuit (i.e., one electrode is positive and one electrode is negative) positioned within the heart is a bipolar arrangement. The bipolar arrangement of electrodes requires two insulated wires positioned within the lead. The lead with such an arrangement is called a bipolar lead. When one electrode is positioned in or about the heart on a lead and represents one pole and the other electrode representing the other pole is the pulse generator, this arrangement is known as a unipolar arrangement. The unipolar arrangement of electrodes requires one insulated wire positioned within the lead. The lead is called a unipolar lead. Unipolar leads are generally more reliable since there are a lesser number of electrodes. Bipolar leads are advantageous since they tend to reject unwanted local pulses and attenuate far field pulses when sensing. When pacing, the electric field is more localized, helping to prevent interference with other leads. The bipolar lead's sensitivity can be used to prevent interference between two devices implanted in the heart. Pulses from a pacing system need to be ignored so they are not used in making defibrillation decisions, for example. A disadvantage of bipolar leads is that they are larger in diameter than unipolar leads since more conductors are required for a bipolar lead.
There are four main types of pulses which are delivered by a pulse generator. Two of the pulse types are for pacing the heart. First of all, there is a pulse for pacing the heart when it is beating too slowly. The pulses trigger the heart beat. The pulses are delivered at a rate to increase the heart rate to a desired level. The second type of pacing is used on a heart that is beating too fast. This type of pacing is called antitachycardia pacing. In this type of pacing, the pacing pulses are delivered initially at a rate faster than the beating heart. In antitachycardia pacing, the rate of the pulses is slowed until the heart rate is at a desired level. The third and fourth types of pulses are delivered through large surface area electrodes used when the heart is beating too fast or is fibrillating, respectively. The third type is called cardioversion. This is delivery of a relatively low energy shock, typically in the range of 0.5 to 5 joule, to the heart. The fourth type of pulse or signal is a defibrillation pulse which is the delivery of a high energy shock, typically greater than 20 joules, to the heart.
The electrodes attached to the lead and positioned in any chamber of the heart can be used to sense the electrical pulses that trigger the heartbeat. Electrodes detect abnormally slow (bradycardia) or abnormally fast (tachycardia) heartbeats. In response to the sensed bradycardia or tachycardia condition, a pulse generator produces pacing or defibrillation pulses to correct the condition. The same electrode used to sense the condition is also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
Some patients require a pacing system having multiple sites in one chamber of the heart for detecting and correcting an abnormal heartbeat. In the past, a common practice for a patient requiring multi-site pacing within one chamber of the heart, would be to provide two separate and different leads attached to the particular chamber of the heart. One lead would be implanted at one site in the chamber. Another lead would be implanted at another site in the chamber. The electrodes on the separate leads would deliver pacing pulses. Typically, the single chamber of the heart receiving multi-site pacing would be the right atrium.
Having two separate leads implanted within the heart is undesirable for many reasons. Among the many reasons are that the implantation procedure for implanting two leads is more complex and also takes a longer time when compared to the complexity and time needed to implant a single lead. In addition, two leads may mechanically interact with one another after implantation which can result in dislodgment of one or both of the leads. In vivo mechanical interaction of the leads may also cause abrasion of the insulative layer along the lead which can result in an electrical failure of one or both of the leads. Another problem is that as more leads are implanted in the heart, the ability to add other leads is reduced. If the patient's condition changes over time the ability to add leads is restricted. Two separate leads also increase the risk of infection and may result in additional health care costs associated with re-implantation and follow-up.
Because of the above problems, there is a need for a single-pass transvenous pacing lead that has electrodes for positioning at multiple sites within a chamber of the heart. A single-pass lead equipped with at least two electrodes would allow for better pacing therapy to a single chamber of the heart. There is still a further need for a single-pass endocardial lead that is easier for a surgeon to implant.